Pain de Cacao
Join Date: Nov 2009
Location: District of Columbia
Device(s): LG G2, Moto G, Galaxy S3, Galaxy Nexus LTE, Nexus 7, HTC Flyer
Thanked 6,034 Times in 3,953 Posts
Do you have a custom recovery installed like Clockwork Mod or TWRP? If so you can do a factory reset in there. If not, to flash the former, download ROM Manager (you don't need the paid version); for the latter, GooManager.
Factory resets from these recoveries won't wipe your SD Card, so if you want to remove any media files, transfer those over manually and then delete. After a factory reset, you'll be prompted to enter in the new Google account and she'll be all set.